The invention relates to a method of and an arrangement for controlling the operation of a hand-held electrical device comprising an axially displaceable output shaft driven by an electric motor via a coupling.
The invention in particular relates to electrical hand-held devices such as screwdrivers and will be explained in detail on a basis of a screwdriver, though it should be understood that the basic principles of the invention are applicable not only to the screwdrivers but may be applied with accompanying advantages to any other hand-held electrical devices, especially such as drills, drill hammers, etc.
In screwdrivers, usually a form-locking coupling, i.e., a claw coupling, is used for transmitting a torque from the electric drive motor. This coupling breaks the form-locking connection between the motor drive and the working spindle when the working spindle, which gradually slides forward during the screwing operation and cooperates with a depth stop, reaches a predetermined thread depth. The claws then disengage from each other, so that the screwing operation ends.
Often such claw coupling is used with an additional device, i.e., such as a slip coupling (disclosed in German patent 3,637,852). This coupling is set to operate in such a manner that it provides for complete separation of the claw coupling elements by increasing the distance between the claw coupling elements immediately at the first slipping of the claw coupling to prevent further mutual slipping of the separate claws, especially in the moment of beginning of declutching, to thereby prevent noise accompanying slippage as well as wear.
This type of control of torque transfer is also used in a power screwdriver according to European patent application No. 90102260.8. In the European application, the torque-limiting slip coupling, which is associated with the coupling elements of the claw coupling located on the side of the driving shaft, includes at least one axial opening in a form of a guide pocket that opens toward the screwdriver tool and has a bottom inclined relative to the longitudinal axis of the driving shaft, and a transverse pin supported on the driving shaft for joint rotation therewith and for engaging the guide pocket.
Another power cut-off screwdriver is disclosed in German Offenlegungsschrift 30 15 423. In this screwdriver, the working spindle stops, when a predetermined limiting torque is exceeded, not as a result of turning-off of the electric motor but due to the coupling declutching whereby further movement of the working spindle beyond a marked position, defined by a pressure point, outward of the guide element can be effected anew. However, a position is then arrived at in which this automatic uncoupling is not possible.
There exists another problem with these screwdrivers when screw elements are fitted in a rapid succession on a holder of a working spindle or output shaft of a screwdriver, and the working spindle, with a strong thrust to insure screwing, is pressed backward to move the claw coupling into the coupling position. Such a method, which is by itself questionable, poses an additional problem for an operator accustomed to working with such screwdrivers when he has to work with a pressure-starting push-in or tachymetric switch, and the drive motor, on fitting a new screw element, is not turned off and the idle speed is not reduced. In this case, the stationary condition of the shaft, necessary for fitting of a screw element, is provided by declutching of the claw coupling. After the screw element is fitted on, the spindle is simply pressed again with the drive motor rotating at high speed (i.e., 3,000-5,000 revolution per min.). Here, the coupling process takes place when the operator presses rather strongly to bring the two claw coupling parts in engagement with each other. Nevertheless, such process cannot prevent poor engagement of the claws because the claws of one coupling part have a zero speed whereas the other coupling part, in this screwdriver, has a high speed. It is difficult to achieve a maximum output with such a screwdriver and one has to put up with considerable noise as the device operates with the maximum working speed or a speed close to the maximum speed though, even with rapid fitting of the screw elements, only 10% of the working time is spent on screwing while the other 90% is spent setting the device and fitting the new screw elements.
A further disadvantage of the prior art screwdrivers lies not only in that, with the maintaining of a high idle speed, a high coupling wear takes place during engagement of the coupling, but also in that the service lives of other elements of the device such as bearings, switches, etc., are also reduced when the device is operated, under normal conditions, with high speed. Such an operation is also accompanied by a disturbing and unacceptable noise.